The present invention relates to a composite article and a method for treating metal foil to prevent or avoid tarnish of the surface of the metal foil. In particular, the present invention relates to a method of forming the composite article including treating the metal foil with an inert silane, titanate or zirconate compound to form a protective film which improves the metal foil""s resistance to tarnish when the metal foil is attached or held adjacent to a metal sheet comprising a different metal.
Copper clad laminates are the basic component of the printed circuit boards used in the electronics industry. In the most common processes, a copper foil is bonded to a prepreg of resin, such as epoxy resin, which has been impregnated into fiberglass by heat and pressure. The copper foil surface that is pressed against the prepreg is typically an uneven or profiled surface with some kind of additional bonding treatment applied to insure that the laminate remains together under normal processing conditions.
The opposing foil surface (the surface not bonded to the prepreg) is typically a smooth surface with various treatments that are aimed at preventing oxidation of the foil and allowing solder wettability and adequate photoresist adhesion. Such coatings may include metals such as zinc or chromium or alloys thereof. A number of such metal treatments are disclosed in U.S. Pat. No. 5,908,544, which is incorporated herein by reference for its teachings with respect to such coatings.
The opposing foil surface is often placed against or attached to a metal sheet comprised of a metal other than the metal foil. Thus, the metal foil may be made of a first metal, such as copper, and the metal sheet may be made of a second metal, such as aluminum. Metal foils are typically very thin and need to be protected from indenting, scratching, bending and folding during shipping, storage and handling prior to and during the lamination of the foil to prepregs. Metal sheets, of a metal which is less expensive and may be discardable, are commonly used for the purpose of providing such protection. In addition, such metal sheets can provide for distribution of heat and pressure in lamination processes, and act as a separator-release plate, or separator sheet, during the lamination of metal foil-prepreg laminates into books, from which PCBs are formed. The metal sheets may be made of any metal, such as stainless steel or aluminum. Aluminum is widely used in the industry today, due to the facts that it is relatively inexpensive and it can be recycled, which facilitates discarding of the metal sheet subsequent to its use. Such metal sheets, and methods of using the sheets with metal foils such as copper foils, are disclosed in U.S. Pat. Nos. 4,875,283 and 5,153,050, and the progeny of each of these patents. The disclosures of both U.S. Pat. Nos. 4,875,283 and 5,153,050 are incorporated herein by reference for their teachings of the use of metal sheets and separator sheets with metal foils.
As disclosed in detail in U.S. Pat. No. 4,875,283, the method thereof makes laminated printed circuit boards of the type having outer conductive metallic layers and at least one inner dielectric layer containing heat curable resin. The metal sheets (referred to therein as xe2x80x9cseparator-release sheetsxe2x80x9d) are formed of aluminum and are coated on both sides with a polymeric resin release material in order to avoid or prevent adhesion between the conductive metal layer, which is generally a metal foil, in particular a copper foil, and the metal sheet. In addition to aluminum as the metal of the metal sheets, other metals may be used for this purpose.
The process of making the laminated printed circuit boards disclosed in U.S. Pat. No. 4,875,283 involves assembling a multi-layer book of circuit boards, one on top of another, in a press lay-up. In the process described in U.S. Pat. No. 4,875,283, a metal sheet is placed in engagement with each outer metallic layer of the board, and the laid-up book is subjected to heat and pressure to cure the resin. Next, at least one of the cured boards is separated from the book with the metal separator sheet remaining in engagement with the outer metallic layer of the separated board. Subsequently, the board may be drilled with the metal sheet attached to form, e.g., conductor lead holes in the board. Thereafter, the metal sheet is removed. The metal sheets may serve to prevent the resin from contaminating adjacent boards in the curing process, and also serve as drill entry and drill backup material in the drilling process, as well as serving as separator sheets in the lamination process.
As described in U.S. Pat. No. 4,875,283, the metal sheets provide protection to the metal foil during the lamination process. In addition, the metal sheet may be used to provide such protection prior to such lamination, such as during processing and handling.
When the metal sheet is attached to a metal foil, such as a copper foil, for use other than the immediate lamination steps described in U.S. Pat. No. 4,875,283, the metal foil is attached with an adhesive, as described in U.S. Pat. No. 5,153,050. When the metal foil is copper foil, the metal sheet is aluminum and a single copper foil is attached to one side of the aluminum sheet, the article is referred to as xe2x80x9cCA.xe2x80x9d When the metal foil is copper foil, the metal sheet is aluminum and a copper foil is attached to both sides of the aluminum sheet, the article is referred to as xe2x80x9cCAC.xe2x80x9d
A problem which has been encountered in the use of metal sheets such as those of U.S. Pat. Nos. 4,875,283 and 5,153,050 with metal foils is tarnishing, oxidation and/or corrosion of the surface of the metal foil facing the metal sheet, which problem types are generally referred to herein simply as xe2x80x9ctarnishxe2x80x9d or xe2x80x9ctarnishing.xe2x80x9d
Thus, a need exists in the art for metal foils which resist the formation of tarnish, oxidation and/or corrosion when in contact with metal sheets.
In one embodiment, the present invention relates to a composite article, comprising a metal foil having a first side and a second side; a protective film of at least one inert silane, titanate or zirconate overlying the first side of the metal foil; and a metal sheet having a first side and a second side, the first side overlying the protective film.
In another embodiment, the composite article further includes a prepreg laminated to the second side of the metal foil.
In one embodiment, the metal foil comprises copper. In one embodiment, the metal sheet comprises aluminum. In one embodiment, the protective film is formed from an inert silane. In one embodiment, the inert silane is propyltrimethoxysilane.
In another embodiment, the composite article further includes a second metal foil having a first side and a second side; a protective film formed of at least one inert silane, titanate or zirconate overlying the first side of the second metal foil; and the second side of the metal sheet overlying the protective film on the second metal foil.
In one embodiment, the first and second metal foils each include a first portion which is adjacent to the metal sheet and a second portion which extends beyond a periphery of the metal sheet. In one embodiment, the second portions of the first and second metal foils are attached to each other. In one embodiment, the first portions of the first and second metal foils are adjacent to, but are not attached to, the metal sheet.
In one embodiment, the present invention relates to an article for use in printed circuit board manufacture, comprising:
a metal sheet of a first metal;
a metal foil of a second metal different from the first metal, a first side of the metal foil attached to a side of the metal sheet, the first side of the metal foil having bonded thereto a protective film having a thickness from about 0.001 microns to about 1 micron on a surface of the metal foil, the protective film formed of an inert silane, titanate or zirconate compound.
In another embodiment, the present invention further relates to an article for use in printed circuit board manufacture, comprising:
a metal sheet of a first metal, the metal sheet having a length and width;
a pair of metal foils of a second metal different from the first metal, a first side of the metal foil adjacent a side of the metal sheet, the first side of the metal foil having bonded thereto a protective film having a thickness from about 0.001 microns to about 1 micron on a surface of the metal foil, the protective film formed of an inert silane, titanate or zirconate compound,
wherein each of the pair of metal foils have a length and width greater than the length and width of the metal sheet and the pair of metal foils are adhered to each other.
In one embodiment, the present invention relates to a method of increasing tarnish resistance of a metal foil of a first metal attached to a metal sheet of a second metal, comprising:
contacting at least one side of the metal foil with an inert silane, titanate or zirconate compound to form a protective film having a thickness from about 0.001 microns to about 1 micron on a surface of the metal foil; and
attaching a side of the metal foil having the protective film to the metal sheet.
In another embodiment, the present invention relates to a method of treating metal foil comprising a method of increasing tarnish resistance of metal foil comprising:
contacting the metal foil with an inert silane compound to form a protective film having a thickness from about 0.001 microns to about 1 micron on a surface of the metal foil.
In another embodiment, the present invention relates to a method of treating metal foil comprising:
contacting a first side of the metal foil with a hydrocarbylsilane solution to form a protective film on a surface of the metal foil, the hydrocarbylsilane solution comprising from about 0.01% to about 10% v/v of a hydrocarbylsilane;
attaching the first side to a metal sheet of a metal other than that of the metal foil; and
laminating a second side of the metal foil to a prepreg.
In yet another embodiment, the present invention relates to a method of treating copper foil comprising a method of treating copper foil comprising:
contacting the copper foil with a solution comprising from about 0.05% to about 5% v/v of an alkyl silane;
attaching a first side of the copper foil to an aluminum sheet; and
laminating the copper foil to an epoxy resin material.
With the present invention, it is possible to provide metal foil which exhibits high tarnish resistance when attached to a metal sheet made of a metal different from that of the metal foil. In particular, the present invention provides a protective coating for application to a metal foil surface that prevents tarnish from forming, particularly on the side of the metal foil which is adjacent a metal sheet and which is not adjacent a prepreg. Thus, a clean, tarnish-free metal foil surface remains after the metal foil has been attached to or held adjacent to, used with, e.g., in lamination, and subsequently detached from, a metal sheet made of a different metal.